Effects of Long Term Unilateral Hearing Loss on the Lateralization of fMRI Measured Activation in Human Auditory Cortex

Abstract

Profound deafness greatly reduces or abolishes neural activity in the auditory nerve (Tucci et al., 1987). This leads to a variety of well documented cellular and molecular effects in the cochlear nucleus on the side or sides of the deafness (Rubel et al., 2003) that are highly dependent on the age at which the deafness occurs (Tierney et al., 1997). In the auditory brainstem, midbrain and primary auditory cortex (PAC) of non-human mammals, a number of changes in neural projection pathways have been described following unilateral deafening (Moore and King, 2003). These seem less dependent on age at the time of deafening. Physiologically, neurons in the midbrain (Kitzes and Semple, 1985; McAlpine et al., 1997) and PAC (Reale et al., 1987; Popelar et al., 1994) have been found to increase their response to acoustic stimulation of the ipsilateral, undeafened ear following unilateral deafening (Figs. 1 A,C). This enhanced physiological response, thought to be a form of neural unmasking, occurs very rapidly following deafening in adults (Mossop et al., 2000) as well as in infants. We initiated the fMRI study described here to explore further the cerebral localization of changes in brain activation following either sensorineural or conductive unilateral hearing loss. However, our initial control studies (Devlin et al., 2003) produced a surprising result (Fig. 1B). We found that the left PAC was more strongly activated by unilateral tone stimulation whichever ear was stimulated. While a 'laterality index' (ratio of contralateral to ipsilateral activation) was much larger for right than for left ear stimulation, stimulation of the left ear consistently produced stronger activation in the left than in the right PAC. In non-primary areas, no significant difference was found between the pattern of activation produced by stimulation of either ear. In light of this result and the report of Khosla et al. (2003), we were intrigued to discover what activation would be observed in participants with hearing loss in either the right or left ear. (PsycINFO Database Record (c) 2017 APA, all rights reserved)